Dalton's Atomic Theory

As per this theory

• All Matter is made up of tiny particles called Atoms, which participate in chemical reactions
  (All types of substances whether compound or mixture are made up of atoms)

• These atoms are indivisible; they cannot be created or destroyed in a chemical reaction
  (They cannot be broken down further)

• All atoms of one element have same mass and chemical properties
  (Example - Mass of an atom of oxygen and the chemical properties it displays will always be same, in any different compounds it is present in)

• Atoms of different elements have different masses and chemical properties
  (Example - Mass of an atom of oxygen will always be different from mass of one atom of Hydrogen)
  
  

• Atoms combine in ratio of whole number to form compounds

  (Example -2 Atoms of Hydrogen combine with 1 atom of Oxygen to form one molecule of water H 2 O. These 2 and 1 are always whole numbers and not fractions)

• In a compound, number and kinds of different elements are constant
  (Example -Water whether obtained from tap or sea will always have 2 atoms of Hydrogen and 1 atom of Oxygen, this ratio of 2:1 will not change)
  

Drawbacks of Atomic Theory

• Dalton viewed atoms as indivisible hard spheres that cannot be broken down further.
  
  
  

But this was not correct. Further studies showed that Atoms are further composed of protons, neutrons and electrons.

Drawbacks of Atomic Theory

• Another finding we say in Dalton's theory was that all atoms of one element have same mass and chemical properties
  

But this was proved to be wrong when isotopes were discovered. Discovery of isotopes proved that atoms of one element can have different masses also.
These are known as Isotopes.

Drawbacks of Atomic Theory

• Dalton’s theory also suggested that Atoms of different elements have different masses.

• However, 2 different elements can also have the same mass.
  These are known as Isobars.
  

Drawbacks of Atomic Theory

Example -

In text Question -Page 33 Q3, Q4

NCERT Question -